FIGURE 6.

Possible reaction pathways for the oxidative half-reaction of CAOs. A, the proposed pathway utilized in the absence of the Cu(I)-semiquinone intermediate for the activation of molecular oxygen (acidic pH). B, the proposed pathway utilized in the presence of an off-copper Cu(I)-semiquinone (neutral pH). C, the proposed pathway utilized in the presence of a copper-bound Cu(I)-semiquinone intermediate (basic pH). Gray boxes represent intermediates for which there are crystal structures from this work and elsewhere (28). PCET, proton-coupled electron transfer; TPQ, 2,4,5-trihydroxyphenylalanine quinone.